1. Field of the Invention
The present invention relates to the mounting of very high bypass ratio turbofan engine units of the forward contrafan type under the wings of aircraft, and to engine pods adapted for this purpose.
In particular the invention relates to a pod for a very high bypass ratio turbofan engine unit of the forward contrafan type suspended from a suspension mast under the wing of an aircraft, said engine unit including an engine casing, a pair of ducted fans having contrarotating propellers disposed at the forward end of said engine unit, and radial flow straightener arms extending radially outwards from said engine casing to the rear of said propellers, said pod comprising an inner cowling of generally cylindrical shape surrounding said engine casing, and an outer cowling also of generally cylindrical shape, said outer casing having an outer side forming the outer wall of said pod and an inner side spaced radially outwardly from said inner cowling and defining therebetween the cold flow path of said engine unit in which said contrarotating propellers and said flow straightener arms are located.
2. Summary of the Prior Art
It is known to mount a turbofan bypass engine of the forward fan type under the wing of an aircraft at the end of a mast suspended from the wing. Such an arrangement is shown in French Patent No. 2,560,854, in which the fan casing is secured to the engine casing by struts and the engine casing is secured at two attachment points to the suspension mast, the first attachment point being situated at the front of the engine casing and the other at the rear of the said engine casing. In this arrangement the mast has a lower part which extends longitudinally and vertically in the cold flow path of the air propelled by the fan, which brings about an undesirable drag during flight.
Moreover, the fan casing, which is integral with the outer cowling of the pod, impedes easy access to the mounting means of the tubojet engine unit during fitting and removal of the engine.
U.S. Pat. No. 4,037,809 discloses a pod for a conventional bypass engine in which the fan casing is integral with the engine casing and is fixed to the mast by fixing means, and in which the rear part of the outer cowling is detachable from the fan casing and is mounted on the mast in a manner such that it can slide rearwards during ground maintenance.
This arrangement applied to a fanjet engine of the forward contrafan type having a very high bypass ratio would not permit easy access to the contrarotating fan propellers or to the reducing gear driving the propellers, since the fan casing is integral with the engine casing. Moreover, the coupling of the rear part of the outer cowling to the fan cowling requires a special joining strip of trapezoidal section.
French Patent No. 2,622,507 relates to a method of mounting a very high bypass ratio turbofan engine of the rear contrafan type under the wing of an aircraft, in which the engine unit is suspended by means of a mast having two arms arranged in a vertical plane containing the longitudinal axis of the engine unit, the upper ends of the arms being rigidly fixed to a structural box of the wing, and the lower ends being rigidly connected to a longitudinal lifting bar arranged between the engine casing and the inner cowling. The engine unit is hitched to the lifting bar at the upstream end of the latter on the engine casing in line with the inlet guide vanes, and at the downstream end on the transition structure arranged between the low pressure turbine and the free turbines driving the contrarotating propellers. In this arrangement both suspension arms are also arranged in the cold air flow path and create an undesirable drag. Ground maintenance is facilitated by the fact that the outer cowling of the pod is attached to the suspension mast or to the structural box of the wing by means of transverse brackets, and this cowling is circumferentially divided into three 120.degree. sectors, the upper sector being integrated into the wing structure, and the two lateral sectors of the cowling having movable cowls which can be raised laterally about longitudinal hinges fixed on the upper sector and which can be locked together at the bottom. Rigidification of this openable outer cowling is achieved by means of radial structural arms mounted at the front and at the rear end of the pod. These arms are rigidly connected to the engine casing and pass through the cold air flow path, thus creating an additional undesirable drag.